This invention relates to an apparatus for providing a carbon-containing coating on optical waveguide fibers.
Optical waveguide fibers are typically provided with abrasion-resistant coatings such as silicone or polyurethane acrylate, for example. These coatings are usually applied to the pristine surface of the fiber during the fiber drawing process. While these coatings provide protection from abrasion, they do not provide adequate protection from corrosion or hydrogen attack.
Various chemicals, including water, can attack a fiber, affecting both optical and mechanical properties thereof. Microcracks in a fiber surface are regions which are more susceptible to such attack, especially when the fiber is under stress. The growth of these microcracks due to chemical attack reduces the mechanical strength of a fiber and may result in static fatigue or sudden failure of the fiber.
If a fiber is exposed to an atmosphere containing hydrogen, the hydrogen will diffuse into the fiber. Such diffusion is detrimental to the optical performance of the fiber. An attenuation increase caused by the diffusion of hydrogen after an optical fiber has been installed may result in degradation of the transmission link which includes the fiber.
The presence of water and hydrogen are of particular concern in optical fiber applications such as underwater cables. These applications often require long-lengths between signal amplification, and there is little or no tolerance for increased attenuation in the fibers during their fiber service life. Also, replacement of fibers which have failed due to chemical attack would be prohibitively expensive.
Various coatings have been developed to provide protection to optical waveguide fibers from chemical attack and to increase the strength of the fiber. These coatings have contained various materials, including carbon, metals and ceramics. See, for example, U.S. Pat. No. 4,512,629 (carbon coating); U.S. Pat. No. 4,592,932 (metallic coating); U.S. Pat. No. 4,118,211 (ceramic coating).
Metallic and ceramic coatings have been used with varying degrees of success with respect to the reduction of strength degradation due to microcracks in the fiber surface. However, such coatings have not proven to be sufficiently impermeable to hydrogen.
Carbon coatings are known to produce water resistant, high strength optical fibers. See, for example, Kao et al. U.S. Pat. No. 4,183,621. Carbon coatings have also been shown to be sufficiently impermeable to hydrogen diffusion. Lemaire et al., "Hydrogen Permeation in Optical Fibres with Hermetic Carbon Coatings", Electronics Letters, vol 24, no. 21, pages 1323-24, Oct. 13, 1988; Lu et al., "Recent Developments in Hermetically Coated Optical Fiber", J. of Lightwave Technology, vol. 6, no. 2, pages 240-244, February, 1988; Lu et al., "Hermetically Coated Optical Fibers", International Wire & Cable Symposium Proceedings, pages 241-244, 1987.
One method for providing an optical waveguide fiber involves exposing a fiber to a carbon-containing reactant gas and decomposing the reactant gas by heating it. The required heat for the reaction may be provided by the temperature of the fiber itself, external heating means, or by some combination of the two. The decomposition of the reactant gas produces a high molecular weight reaction product, which forms the desired carbon layer on the fiber, and reaction by-products. The reaction by-products can be either high molecular weight dry particulate matter similar to carbon black, or low molecular weight oily droplets which solidify to a gummy or glassy material. Also, some portion of the reactant gas may remain unreacted.
Low molecular weight reaction by-products are preferentially formed at lower temperatures. High molecular weight reaction by-products are rapidly formed at temperatures above about 150.degree. C. These high molecular weight reaction by-products can be formed as reactant gas flows along with the fiber through the reactor and is exposed to temperatures in the vicinity of the fiber which are higher than 150.degree. C.
Prior art processes and devices for manufacturing optical fibers with carbon coatings have encountered various problems. Alignment of the reactor, in which the carbon coating is applied to a fiber, with the other components of the fiber drawing apparatus is necessary for process stability and repeatability. Prior art devices which have disclosed the material used in construction of the reactor have disclosed the use of quartz or silica tubes. See, for example, Oohashi et al. U.S. Pat. No. 5,037,464; Ishiguro et al. European Patent Publication No. 0,374,926; Schultz et al. U.S. Pat. No. 4,735,856; Evans et al. UK Patent Application No. 2,156,858. Due to the complex shapes required in the reactor design, fabrication of the reactor using quartz or silica lacks dimensional repeatability which adversely affects the ability to initially align the reactor on the fiber drawing apparatus and to maintain alignment during the fiber drawing process.
Additionally, quartz or silica reactors provide insulating properties, elevating the temperature within the reactor and accelerating the formation of undesired reaction by-products. As a result, the rate of formation of high molecular weight reaction by-products is greater than the formation of low molecular weight reaction by-products. High molecular weight reaction by-products may build up within the reactor and impinge on the fiber, damaging the carbon coating and possibly the fiber itself. Because these reaction by-products build up over time, the ability to produce long lengths of coated optical fiber is reduced. In some cases, entire production lots of greater than 100 km of fiber are rejected to ensure the quality of the coating if build up of high molecular weight reaction by-products is detected within the reactor.
Bennett et al. U.S. Pat. No. 5,152,817, to be issued on Oct. 6, 1992, is assigned to the Assignee of the present application. Bennett et al. discloses an apparatus for providing long lengths of optical waveguide fiber with a carbon-containing coating without the build up of high molecular weight reaction by-products within the reactor.
The apparatus of Bennett et al. is shown in FIG. 1. It consists of a combination of an upper isolation chamber 1, reaction chamber 2, receiving chamber 3, and lower isolation chamber 4. Fiber 5 enters the apparatus at fiber inlet 6 and exits through external fiber exit 7. Reactant gas is introduced at inlet 8. Reaction by-products are exhausted through external fiber exit port 7 or through outlet pipe 9 which can be optionally provided. Shield gas is introduced to upper isolation chamber 1 and lower isolation chamber 4 through shield gas inlets 10 and 11 respectively. The inside diameter of external fiber exit port 7 is the same as the internal diameter of internal fiber exit port 13. In a preferred embodiment, the portion of reaction tube 14 within chamber 2 may be perforated, as shown, to evenly distribute the reactant gas radially around fiber 5.
The length and diameter of reaction tube 14 of Bennett et al. are selected to ensure adequate coating thickness and to reduce the build up of high molecular weight reaction by-products inside the reactor. The preferred length is about 5-6.5 cm for an inside diameter (ID) of 1 cm. However, even with these preferred dimensions, oily reaction by-product may build up on the interior surfaces of reaction tube 14. Bennett et al., col. 7, lines 2-6, Detailed Description. Reaction by-products will deposit on the interior walls of receiving chamber 3. The inside diameter of receiving chamber 3 must be at least about 1 inch (2.5 cm) and the length at least 4 inches (10 cm) for the build up of reaction by-products on the walls of receiving chamber 3 to present no problem in the fiber drawing or coating processes. Bennett et al., col. 7, lines 16-21, Detailed Description.
Even with these design considerations, there is some build up of reaction by-products at opening 12 of internal fiber exit port 13 in Bennett et al. First, an oily low molecular weight reaction by-product will deposit near opening 12. This oily film will solidify over time. High molecular weight particulate reaction by-products will then tend to adhere to the oily film. These particles will serve as sites at which additional reaction by-products will accumulate. This build up can cause damage to the carbon coating or the fiber itself. Even if this build up does not apparently damage the coating or the fiber, detection of the build up will result in rejection of the fiber drawn to ensure the quality of the coating and the fiber. Therefore, even a slight build up is considered unacceptable.
In another embodiment, Bennett et al. discloses a reactor where the bottom surface of the receiving chamber is angled downwardly away from opening 12 through which the fiber exits the receiving chamber. This angle is disclosed in a specific example as being 50.degree. with respect to the fiber axis. Bennett et al., col 10, lines 18-22, Detailed Description. The purpose of this angled surface is to prevent any oily reaction by-products which may deposit on the bottom surface of the receiving chamber from flowing toward opening 12 before that deposit solidifies. Bennett et al, col. 10, lines 39-42, Detailed Description.
The reactor in Bennett et al. is made of glass (typically, "PYREX.RTM.") except for upper isolation chamber 1 and reaction chamber 2, which are made of aluminum. After a preform is drawn into fiber, the reactor is removed from the drawing apparatus and the glass portion is heated to about 900.degree. F. (480.degree. C.) for a period of about four hours in an oxygen-containing atmosphere to burn off any reaction by-products which may have deposited on the surfaces of the reactor.
Glass reactors have been used instead of metal reactors for several reasons. First, fiber drawing speeds are often less than 6 meters per second. At these speeds, the temperature of the fiber may be reduced to a level at which the desired reaction will not occur unless the fiber is either enclosed by an insulating material such as glass or some means of auxiliary heat is provided. Second, during development, visual analysis was required to determine any process parameter changes which were needed to stabilize the carbon coating reaction. Furthermore, because alignment of the carbon coating apparatus was critical to the process, visual alignment was deemed necessary. These two requirements necessitated the use of glass reactors. Also, a metal reactor would deform at the temperature used for burning reaction by-products off the reactor walls as described above. If the burn off method is used with a metal reactor, it would require lower temperatures for unacceptably longer periods of time than are used for burning off deposits on glass reactors. For example, an aluminum reactor exposed to an oxygen atmosphere at 750.degree. F. (400.degree. C.) for a period of ten hours still exhibited some residue from the build up of reaction by-products.
Jochem European Patent Publication No. 0,393,755 discloses a method of manufacturing an optical fiber with a coating wherein the temperature of the reactor walls is below 800.degree. C. The restriction on the maximum temperature of the reactor walls is designed to reduce the build up of reaction by-products on the reactor walls. The reactor "may comprise an insulated wall or a heating device . . . in order to preclude that the glass fibre cools too rapidly." col. 4, lines 20-23. Jochem only discloses reactor wall temperatures ranging from 600.degree. to 900.degree. C. in a specific example. col. 7, lines 10-29. However, even at 600.degree. C., we believe significant amounts of reaction by-products will be deposited on the reactor walls.